It is common practice to screen targets, including people, freight, luggage and other items, to detect the presence of illicit substances. Illicit substances include, for example, narcotics, explosives, chemical warfare agents, biological warfare agents, nuclear or radiological agents, toxic industrial chemicals or waste, and controlled or contraband items such as tobacco.
The presence of explosives on a person's body, clothing and hair can be detected by sensing microscopic particles located on the person or vapors emitted from concealed threat materials on the person's clothes. To detect illicit substances, various techniques may be used, such as physical search, identifying anomalies on a person's body by detecting changes in the dielectric constant within a defined space, and electromagnetic imaging using technologies such as x-ray back scatter, millimeter-wave, or low-power microwave.
Some techniques offers a high probability of identifying presence of explosives, such as physical search, but suffer from being invasive and time-consuming. Others offer lower probability of detection and may be too revealing when applied to a person, raising the issue of invasion of privacy.
Existing trace chemical detection portals (TCDPs) sample the air surrounding a person who is inside the portal, collecting dislodged particulate matter on a single pre-concentrator (usually a large diameter mesh) at high flowrate. The mesh is heated to vaporize the sample into a secondary mesh or directly into an analyzer. The analyzer is usually an ion mobility spectrometer or a more sophisticated quadrupole ion trap, time-of-flight mass analyzer.
TCDPs may be open design or closed door design. In both approaches, fans in the ceiling typically blow downward over the body of a person inside the portal and air jets in the sidewalls provide agitation to remove particulate matter from the person. Explosive vapor and particles are transported in the downward airflow to an exhaust slot near the floor and then through a two-stage pre-concentrator. This design is used in multiple commercially available portals. In another approach, the air is moved upward, relying on the temperature of the human body to generate a convection plume, which causes vapor around the body to move upward to a collection system.
Other designs employ a tunnel that a person walks through and which samples the air carrying any dislodged particulate matter and, vapor while the person walks through the tunnel. Ceiling fans may pull the air upward towards pre-concentrator tubes and into a fast gas chromatography (GC)-electron capture detector.
Because existing designs use a single pre-concentration and analysis module that handles all samples of air ingested by the exhaust slot, including debris such as hairs, they suffer from various problems. The collection of debris, hairs, fibers, and dust eventually saturates the primary mesh of the single large pre-concentrator, reducing sensitivity to explosives and other illicit substances, and introducing major contamination to the analysis system. This results in the requirement for frequent cleaning of the meshes and ducts, and results in the systems working sub-optimally much of the time. In some systems, periodic replacement of the pre-concentrator mesh assembly is required, making field repair costly and laborious.
The need for powerful compressors and blowers to drive particles though the interior to the exhaust slot and the use of a large single centralized pre-concentrator and detection modules results in bulky portals are very difficult to transport between facilities and relocate within a facility.
They also require a large amount of power for their compressors and blowers and have specialized installation and power requirements. They employ heavy and bulky compressors to generate the air jets, and typically use a large blower cable of blowing over 20,000 liters per minute. The noise generated by the compressors and blowers also results in the need for noise reduction measures to be taken in the design, which adds to the bulk and cost of the units.